Heat oven to 350 degrees F. Coat two 8 x 2-inch round cake pans with nonstick cooking spray.

Heat chocolate in microwave oven on HIGH 30 seconds; stir. Heat another 30 seconds until melted. Sift flour, baking powder, baking soda and salt onto a sheet of waxed paper.

Beat butter in large bowl until creamy. Beat in brown sugar. Add eggs, one at a time, beating after each addition. Add vanilla and melted chocolate. Alternately beat flour mixture and milk into butter-chocolate mixture. Pour into prepared pans.

Bake at 350 degrees F for 30 to 33 minutes or until layers spring back when pressed. Cool in pans on rack 10 minutes. Turn layers out onto racks; cool completely.

Frosting: Beat yolks in bowl until very thick. Slowly beat in 1 cup of the sugar, 1 tablespoon at a time, over 5 to 7 minutes until light in color. Beat in milk. Add to saucepan along with remaining 1 cup sugar and butter. Bring to a boil over medium heat, whisking occasionally (12 to 15 minutes). Boil, whisking, 1 minute. Remove from heat. Chop chocolate; stir in until smooth. Transfer to bowl. Refrigerate 30 minutes.

Beat frosting on medium speed 4 minutes until light and fluffy. Immediately frost cake: 1 cup to sandwich layers, 1-1/2 cups for sides and 1/2 cup for top.